The present invention relates to an image forming apparatus using optics including a lamp and, more particularly, to an image forming apparatus capable of operating in an enlarge mode even when the voltage applied to a lamp exceeds a predetermined limited voltage.
A copier, facsimile transceiver, laser printer or similar image forming apparatus of the type having optics including a lamp is extensively used today. This type of image forming apparatus controls the voltage to the lamp in matching relation to the potential of a latent image electrostatically formed on an image carrier, e.g., a photoconductive element and representative of a reference density pattern having a reference density. Hence, as the photoconductive element itself deteriorates due to aging, the voltage to the lamp is sequentially controlled to higher one. This brings about a problem that the lamp is deteriorated by the high voltage and has the life thereof reduced. When the lamp breaks due to the deterioration, an extremely high voltage (1 kilovolts to 2 kilovolts) is applied to the lamp itself to cause it to burst. This is not only dangerous but also causative of a failure due to the pieces of the lamp scattered around in the apparatus.
To prevent the lamp form breaking, there has been proposed an image forming apparatus having a voltage limiting unit for limiting the voltage to the lamp to below a first set voltage, an alarm unit for producing an alarm on determining that the voltage to the lamp has reached a second set voltage lower than the first set voltage, and a unit for changing the second set voltage. Implementations relating to this kind of apparatus are disclosed in Japanese Utility Model Laid-Open Publication (Kokai) Nos. 68672/1988 and 98561/1988 and Japanese Patent Laid-Open Publication No. 212930/1988.
However, the problem with the apparatus of the type limiting the quantity of light to issue from the lamp in terms of the voltage to the lamp is that, as the lamp deteriorates, it is apt to suddenly turn off in an enlarge mode although the voltage to the lamp may have some margin relative to the set value in a 1:1 mode. This problem is ascribable to the fact that since an enlarge mode needs a greater quantity of light from the lamp than in a 1:1 mode, the voltage to the lamp is automatically increased in the enlarge mode to illuminate the surface of the photoconductive element by a greater amount of light. In such a condition, therefore, images cannot be formed in the enlarge mode. While the set voltage of the lamp may be simply shifted up to eliminate such a problem, this approach does not fully settle the problem since an excessive voltage to the lamp would increae the load on the lamp to thereby quicken the breakage of the lamp. Although the lamp may be replaced in the above condition, replacing the lamp at such a stage of is not desirable from the economy standpoint since the lamp is still usable in the 1:1 mode.